An optical filament is a light beam which propagates at high intensity and small radius for long distances, beyond the Rayleigh length, zR, the beam distance for normal diffraction-driven divergence. Herein, for laser pulses, we refer to the beam irradiance, defined as the pulse energy divided by the quantity (beam area×pulse duration), as the beam or pulse “intensity”. Optical filaments generated by laser pulses have been observed in water.
The study of such underwater optical filaments, including determination of optimal generation parameters, maximum propagation length, and filament plasma lifetime, is a focus of ongoing research by scientists at the Naval Research Laboratory, including the inventors of the present invention. See Helle, et al., “Underwater Laser Filamentation and Guiding of Electrical Discharges,” contributed talk at 2011 IEEE Conference on Plasma Science, Chicago, Ill. (2011).
It is believed that the diameter of such an optical filament in a given medium is a function of wavelength and pulse duration. For example, 70 femtosecond, 800 nm wavelength optical filaments in air have an observed diameter of 70 microns, see Y.-H. Chen, S. Varma, T. M. Antonsen, and H. M. Milchberg, “Direct Measurement of the Electron Density of Extended Femtosecond Laser Pulse-Induced Filaments,” Phys. Rev. Lett. 105, 215005 (2010), while 5 nanosecond, 532 nm underwater optical filaments have an observed diameter of 100 microns, see M. Helle, T. G. Jones, J. Peñano and A. Ting, “Formation and propagation of meter-scale laser filaments in water,” submitted to Opt. Lett., November 2012.
Both air and water are transparent to a range of wavelengths, enabling an intense laser beam to propagate many tens of meters. In addition, both media have nonlinear dielectric response with respect to the optical field intensity. For example, water exhibits a positive nonlinear index of refraction (Kerr effect) above a certain laser intensity threshold. In addition, water undergoes photoionization when subjected to a laser pulse above a separate ionization intensity threshold. See U.S. Pat. No. 7,260,023, “Remote Underwater Laser Acoustic Source,” (“Jones '023”) which has at least one inventor in common with the present invention and which is hereby incorporated by reference into the present disclosure in its entirety.
The threshold laser power needed to induce the Kerr effect is
            P      NSF        =                  λ        2                    2        ⁢        π        ⁢                                  ⁢                  n          0                ⁢                  n          2                      ,where λ is the laser wavelength, n0 is the linear index of refraction, and n2 is the nonlinear index of refraction, so that to lowest order in the laser intensity, n=n0+n2I. For visible wavelengths in water, PNSF is of the order of 1 megawatt (MW). See '023 patent, supra.
The threshold irradiance needed to cause underwater photoionization is pulse length dependent, and ranges from approximately 1010 Watts per square centimeter (W/cm2) for ns pulses to more than 1013 W/cm2 for femtosecond (fs) pulses. Id.
Although the mechanisms of underwater filament formation have not been definitively determined, a leading theory is that underwater optical filaments are analogous to filaments in air, and are formed as a result of Ken-induced beam self-focusing balanced by ionization-induced beam defocusing and diffraction. The underwater filament so generated can propagate through the water for many Rayleigh lengths.
According to the leading theory, these underwater optical filaments have an extended low energy plasma associated therewith. When the optical filament plasma is heated by another laser pulse in accordance with the present invention, an extended energetic underwater plasma can form which is useful for generating underwater vapor channels and guiding longer range underwater electrical discharges. See U.S. Pat. No. 8,941,967 B2 entitled “Underwater Laser-Guided Discharge,” (“Jones '967”) which has at least one inventor in common with the present invention and which is hereby incorporated into the present disclosure in its entirety. Such discharges could enable a new class of undersea weapon, or could be useful for micromachining, potentially combining desirable features of underwater femtosecond laser machining and underwater electric discharge machining.
An especially important potential application of extended underwater plasmas for the Navy is the generation of shaped energetic underwater plasmas suitable for long-duration underwater acoustic pulse generation. Longer duration acoustic pulses suffer less ultrasonic attenuation, and thus have much greater acoustic range, making them useful for applications like long range sonar and long range acoustic communications.
However, there is presently no technique to remotely generate a spatially extended underwater plasma. One currently available technology for generating extended underwater plasmas, available from Applied Acoustic Engineering Ltd, Marine House, Marine Park, Gapton Hall Rd, Great Yarmouth NR31 0NB, U.K., employs underwater spark gaps, known as “sparkers” when used for acoustic generation. Another currently available technology employs underwater wire discharges, see Y. E. Krasik, A. Grinenko, A. Sayapin, S. Efimov, A. Fedotov, V. Z. Gurovich, and V. I. Oreshkin, “Underwater Electrical Wire Explosion and Its Applications,” IEEE Transactions on Plasma Science 36, 423 (2008). However, both underwater spark gaps and underwater wire discharges require hardware in the water, and therefore cannot generate acoustic signals remotely. Furthermore, an underwater wire discharge is a single-shot device; because the wire is vaporized during each pulse, repeated wire replacement is required for multiple-shot operation.
Single laser pulse techniques for remotely generating energetic underwater plasmas have been developed at NRL, see Jones '023, supra; see also T. G. Jones, et al., “Intense Underwater Laser Acoustic Source for Navy Applications,” Invited talk at 157th Meeting of Acoustical Society of America, Portland, Oreg., May, 2009. However, the plasmas generated by such techniques have only reached lengths of about 5 cm and so are not suitable for generation of long-duration underwater acoustic pulses or for guiding long-range underwater electrical discharges.